I've spent a lot of the last two to three years experimenting with incorporating strength training into a fairly rigorous running program. I've done full-on Crossfit, modified Crossfit and all sorts of other combinations of strength/circuit work that mix running with strength training. Honestly, I don't know what I'm doing. A lot of what I've tried has not worked at all. But bit by bit...through trial and error...and a lot of stumbling along...I'm starting to notice some patterns.Here's an interesting article on using strength training to improve running: http://breakingmuscle.com/running/heavy-lifting-improves-running-economy-in-masters-athletesThe article cites a recent study entitled “Concurrent Strength and Endurance Training Effects on Running Economy in Master Endurance Runners,” Journal of Strength and Conditioning Research, 27(8), 2013., by MF Piacentini, et. al.The study essentially found that Masters Athletes could improve running economy with a strength program that had the following characteristics:

Low amount of repetitions for each lift/exercise (3-4 reps). Did 4 sets.

Weights that were 85-90% of 1RM (maximum for one repetition).

Long recovery between sets (3-4mins)

In the study, they used simple stuff like squats, lunges, leg press, bench press, lat pull-downs, cable push-downs, tricep extensions and bicep curls. These would not be my choices but they worked well enough for the study.The main point to take away is that heavy weight, few reps, long recoveries worked for runners...and did not put on mass. I used to think that heavy weights put on mass, but I was wrong. Heavy weights with low reps seem to build strength without the mass. The strengh gains seem to be mostly neurological. This jives with my experiences so far.If you want to do well at most endurance sports, you want to stay light so you generally want to avoid any strength training that puts on mass. Breaking Muscle has a good article on the topic. You'll notice that this article also confirms that you get strength gains...without a corresponding mass gain...when you do relatively heavy weights for low repetitions with lots of recovery. Interestingly, the article also claims that you can increase muscular endurance...which is entirely different from strength...by doing light loads of 40-60% of 1RM for many reps...and that this will also not increase mass. I'm not sure this helps runners though as they are already doing loads of endurance work.So if you are an endurance athlete that is trying to blend strength work into a running program, it seems that going heavy with low reps and lots of recovery may be the way to go. When I do these kind of workouts, they do not beat me up at all. I do not set any records, but I use weights that are heavy for a runt like me.Phil Maffetone's approach may be the best of all for endurance athletes when it comes to resistance training.One thing I'd also suggest...especially if you do Boot Camp classes, Crossfit, Body Pump, or any supplementary strength and conditioning work...and you do not want your running to suffer...Do Not Go to Failure. It's been my experience that this sets you back more than anything...without much benefit.If you are just starting out with any of this...Take 'Er Easy...baby steps...one, two, three...All the best...